bigs

I know. I shoulda included a smilie.

Hey, how do these smilie thingeys work, anyway?!

Randy Carter

Bigs, they work off the PFM board.

bigs

What're you drinking tonight??!!

BC

Just combine a : and an ) and it makes

bigs

OK...

Back on topic here.

Does anyone know exactly how the brake pad sensors do their thing?

AO

It's a simple circuit. Basically it's an open circuilt but when the pad wears down the sensor gounds out on the rotor and completes the circuit and the light comes on. My pad sensors are long gone, just need to inspect the pads each time I have the wheeels off.

Shark Attack

However no one answered you original question. They work Great!

Seriously though:
my knowlegde comes from a corvette. the general op is the same but the location of parts may be different.

Basically you have magnets located in the hub of the wheel. The ABS sensor is a small coil. If you remember science class, when you pull a magnet across a coil it forces the electrons to move. thus moving electrons IS electricity... So the sensor (coil) gets mounted in the knuckle, it rides very close to the magnets. As the wheels turn, it drags the magnets across the coil, thus telling the ABS CPU the wheel is in fact turning.

thus when the voltage drops from the wheel being stopped, the cpu is told the wheel has stopped turning. The cpu then tells a pump "hey the ******* behind the wheel is in a little girl panic... can you help him out and release some pressure on the brakes. it does so with a pump, this is why you feel the pedal push back at you when the ABS is engaged.

Their is more to it.... how does the car know when it should be stopped VS the wheels locked up? Good question. If i remember right the Vett had a sensor that would be tripped in a hard brake situation kinda like when your seatbelts lock up when you brake hard.

bigs

Nice explanation! Thanks!

ew928

A.O.,
I thought the pad wear sensor was a wire loop that sits inside the brake pad.
It's normally closed circuit (continuity). When pad wears down enough to expose wire, rotor will chafe through the wire and creates an open circuit. That triggers the warning.
On my car (88 S4), A missing pad sensor will trigger the warning and I had to short the pad warning connector to keep the warning from triggering when I had a bad sensor.

bigs,
The neat thing about ABS is that the ABS sensor will modulate brake force to the wheels that are turning faster than the slower wheels (ones presumably with more traction) to try and maximize braking force on all 4 wheels (get all wheels to just before lockup).
ABS will try and keep the front wheels from full lockup so one retains steering in hard braking. Bad thing in the early days of ABS is that driver computer (brain) tends to panic brake and steer away from obstruction and some ended up braking and steering themselves off the freeway/highway.

fabric

Exactly. So if the pad is just dangling, but still plugged into the junction box, it will not set of the warning. The circuit needs to be broken for the dash warning to come up.

bigs,
The neat thing about ABS is that the ABS sensor will modulate brake force to the wheels that are turning faster than the slower wheels (ones presumably with more traction) to try and maximize braking force on all 4 wheels (get all wheels to just before lockup).
ABS will try and keep the front wheels from full lockup so one retains steering in hard braking. Bad thing in the early days of ABS is that driver computer (brain) tends to panic brake and steer away from obstruction and some ended up braking and steering themselves off the freeway/highway.[/QUOTE]


I believe the 928 is a 3 channel system, and the rear wheels are treated as 1 channel, so there's no individual management of the wheels in the back, so you have to a slipping condition in both wheels before ABS kicks in, so one rear wheel can lock up. I had looked at this before, thinking the later models were upgraded to 4 channel, but hadn't seen anything stating this.

69gaugeman

correct

Unless they changed it on newer ones it is the opposite. It is a closed circuit that breaks when it wears through, creating an open circuit. To defeat the system tie the two wires together. I haven't seen one that operates as an open circuit yet, but there might be a first time for everything.

bigs

If the end plug has come out of its housing, wouldn't that break the circuit and thus trigger a warning?

ew928

Yup. ^^^^^^

When my pad sensor corroded itself to smithereens, I got some spare wire and bridged the connector (shorted the 2 plug connectors) till I could get a new pad sensor in there.
Or the damn yellow light comes on.

Goal in life. To extinguish ALL the yellow warning lights on the dash.

bigs

So now I'm still scratchin' my head as to why I never got a dash warning light...

James-man

The brake pad wear sensor clips to the middle of the brake pad. If it comes loose, it is still intact. The little black "sensor" has to be worn down to smithereens from direct scraping on the rotor in order for the electrical circuit to be broken. When the circuit is broken, the warning light comes on. If the wear sensor popped out from the brake pad, it is still intact and will not trigger a warning. The little black square is not a connector. It is a wearable material that will start wearing away when one of the brake pads gets too thin. Once it wears to the point where the electrical wires inside are exposed, they will break and the dash light comes on.

One of my wear sensors is zip tied away from everything so I don't have to mess with it every time I swap pads (which is at least several times per year). I do not get warning because the sensor is still electrically intact while being located someplace other than where it should be.